Given a string, find the length of the longest substring without repeating characters. Examples: Given "abcabcbb" , the answer is "abc" , which the length is 3. Given "bbbbb" , the answer is "b" , with the length of 1. Given "pwwkew" , the answer is "wke" , with the length of 3. Note that the answer must be a substring , "pwke" is a subsequence and not a substring. Ref: https://leetcode.com/problems/longest-substring-without-repeating-characters/